President Obama walks back to the Oval Office after he made a statement about Syria in the Rose Garden at the White House in Washington on Aug. 31. / Charles Dharapak, AP

by Paul Singer and Aamer Madhani, USA TODAY

by Paul Singer and Aamer Madhani, USA TODAY

President Obama's bid to get congressional support to use military force in Syria received a boost Monday as Republican Sens. John McCain and Lindsey Graham said they have more confidence the White House is developing a better strategy for dealing with Syria.

McCain and Graham are key votes Obama will need to win Senate approval for the United States to launch missile strikes againstSyria in response to an Aug. 21 chemical weapons attack that killed more than 1,400 people.

Obama said Saturday he had concluded the United States should launch an attack in response to the attack, but he said he wants approval first from Congress.

McCain and Graham have jointly expressed concerns that a military strike should be part of a broader strategy in Syria, not simply a random attack to punish the regime.

After meeting with Obama Monday, they both said they believed the White House is developing a strategy that would weaken the regime of President Bashar Assad and boost Syrian opposition forces - though they said Obama has more work to do to explain this plan.

"We still have significant concerns," McCain said, "but we believe there is in formulation a strategy to upgrade the capabilities of the Free Syrian Army and to degrade the capabilities of Bashar Assad. Before this meeting, we had not had that indication."

McCain of Arizona repeatedly said a congressional vote rejecting the use of military force would be "catastrophic" to U.S interests and would destroy the credibility of the nation in the eyes of both allies and adversaries. Graham of South Carolina said, "If we don't get Syria right, Iran is surely going to take the signals that we don't care about their nuclear program. ... If we lost a vote in Congress dealing with the chemical weapons being used in Syria, what effect would that have on Iran and their nuclear program?"

Both senators criticized the administration for lacking not having a clearer strategy in Syria before now.

In Syria Monday, Assad told a journalist with the French newspaper Le Figaro that any attack risked opening a wider war in the region.

Syria has challenged the United States and France to provide proof to support their allegations that Damascus has used chemical weapons, Assad said, but that the leaders of both countries "have been incapable of doing that, including before their own peoples."

McCain and Graham, however, said they had no doubts about Syria's use of chemical weapons. Their meeting Monday was part of a major lobbying push by the White House.

House Democrats were briefed on a conference call by National Security Adviser Susan Rice, Secretary of State John Kerry, Defense Secretary Chuck Hagel, Director of National Intelligence James Clapper and Chairman of the Joint Chiefs of Staff Martin Dempsey.

Kerry said Sunday the administration has more proof Syrian troops used the nerve agent sarin in the Aug. 21 attack.

Tuesday will be another busy day of lobbying for the White House. Obama will meet with chairs and ranking members from key national security committees, including the Senate Armed Services Committee, Senate Foreign Relations Committee, Senate Select Committee on Intelligence, House Foreign Affairs Committee, House Permanent Select Committee on Intelligence and House Armed Services Committee.

The Senate Foreign Relations Committee will conduct a hearing on the issue Tuesday afternoon in which Hagel, Kerry and Dempsey are likely to testify.